Conventional lighting historically has used incandescent and fluorescent bulbs, but recently with the invention of the blue LED, has started to use LED lights. The initial cost of the LED light is high, but over time the power savings can reduce the overall cost of lighting substantially. Part of the high initial cost of a power efficient LED light is the special electronics necessary to create a constant current to the LEDs from a power source.
Most LED lights today consist of multiple LEDs connected together in series and/or parallel, and are driven by a switching power supply connected to the AC mains. Example circuits can be found on websites from many semiconductor manufacturers.
The LEDs in a light can be any color or any combination of colors, including white. White LEDs are typically made with a blue LED covered in some type of yellow phosphor. Much of the blue light from the LED is absorbed by the phosphor and re-emitted at lower frequencies corresponding to green, yellow, and some red colors. Some advantages of this approach include low cost and more natural continuous spectrum light. Some disadvantages include low efficiency due to losses in the phosphor, a bluish color from the LED, and reduced reliability due to degradation of the phosphor.
To overcome the lack of energy at the red end of the spectrum, some manufacturers produce a two color overhead LED lamp that includes strings of red LEDs together with strings of phosphor coated blue LEDs, which produces a good cost/performance compromise for many applications.
The ideal LED light from a color spectrum perspective would include many different colored LEDs operating at different power levels to produce a rough approximation of either incandescent or sun light. For example, the combination of red, yellow, green, and blue could be used as a set number of colors. Although this approach should have a good spectrum and be more energy efficient and reliable, control of the relative power levels in each color is difficult and expensive in practice today.
In one lighting solution, a white plus red LED lamp includes two chains of 6 white LEDs, and one parallel/serial combination of 30 red LEDs for a total of 36 LEDs. This solution includes a photodiode and a thermistor to maintain color. As the reflected light to the photodiode decreases, the output power from the red LEDs is increased in compensation. This circuit uses three different LED driver circuits and uses a single silicon photodiode to monitor light power from which the LED current is adjusted. U.S. Published Patent Application No. 2008-0309255 describes the photodiode selectively measuring part of the light spectrum and adjusting the red current based on average power, which provides an indication of the spectrum being produced but with low resolution.
Lighting control systems vary in complexity from a simple dimming switch to centrally controlled building wide networks. Like a simple dimming switch, most complex lighting systems use wires to control each lamp. Additionally, complex systems comprise special electronic modules attached to each lamp to communicate between lamps and to a central controller. Recently, wireless lighting control systems have been introduced the use radios to communicate information according to protocols such as ZigBee, which eliminate the cost of wires but add the cost of the radios and the protocol stack.
Conventional light dimming switches use a triac circuit that only allows the mains AC voltage to be applied to an incandescent light during part of the cycle. For instance, when set at half power, the voltage signal that passes through to the light is zero for the first 90 degrees of the sinusoidal voltage, jumps to the peak amplitude and follows the sinusoid down to zero for the second 90 degrees, stays at zero for the next 90 degrees, and finally jumps to the negative peak voltage and follows the sinusoid back to zero. This approach is a cheap and effective way for a consumer to dim a resistive incandescent bulb.
Although the triac dimmer reduces power consumption in the light bulb, it does not reduce the power that the utility company must produce. Power companies produce current that is in phase with the voltage. As the voltage increases, the current increases. If the entire load on a power generation plant consisted of lights dimmed 50% with triacs, the current produced during the first half of the positive and negative cycles would not go to the bulbs, but it would have to go somewhere. The utility must generate the same amount of power whether the lights are full on or dimmed and must deal with potentially dangerous transients on the grid.
The light from an LED can be reduced by either reducing the drive current or reducing the time that the current is applied through what is called pulse width modulation (PWM). The current is turned on and off at a rate faster than the eye can see, with the duty cycle proportional to the light output. Since the wavelength of light produced by an LED changes with drive current, PWM dimming is preferred. When replacing an incandescent light with an LED light, an existing triac dimmer still adjusts the power supply to the light. To enable PWM dimming, the LED light circuitry must filter the power supply, detect the duty cycle of the supply, and adjust the PWM duty cycle accordingly, which adds cost and complexity.
Lighting control systems sometime provide remote controllers using RF or infrared communication to allow the brightness of individual lamps or groups of lamps to be adjusted. Such an approach eliminates the need for such dimmer switches, but requires lamps to have the circuitry necessary to communicate with remote controllers to communicate with the lighting control system.
Daylight harvesting is a term used in the lighting industry to describe actively adjusting the brightness of lamps in response to changes in ambient light. For instance, lamps near windows of a building during the day do not need to produce as much light as lamps in an interior corridor. Photo-sensors placed throughout a building typically measure the ambient light at strategic places and the lighting control system determines the brightness needed from each lamp and communicates such information to each lamp.
Scheduling is a term used to describe the adjustment of lamp brightness based on time. For instance, at night when a building is empty some of the light may automatically turn or the brightness of some lights may be reduced to save energy. Lighting control systems typically provide such functionality from a central location by sending digital control messages to each lamp with instructions on what brightness to produce.
Occupancy sensors or motion detectors can save substantial energy by only turning lamps on when people are present. In a typical lighting control system, an occupancy sensor may be located near a door and will alert the lighting control system when a person is present. The lighting control system then typically sends messages to the lamps indicating the desired brightness.
Although lighting control systems that implement functions such as remote control, daylight harvesting, scheduling, and occupancy sensing can save substantial energy, the initial cost of such systems can be prohibitive high, particularly for existing buildings without the necessary wiring and infrastructure.